# Newton's Third Law

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Thread starter 1 month ago
#1

I know that B is the force exerted by the ground on the Child,
C is the force exerted by the Child on the Earth
and D is the force exerted by the Earth on the Child.

So, isn't A, the force exerted by the Child on the ground?
and how' it the force exerted by the Ground on the Child?
Last edited by Aleksander Krol; 1 month ago
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1 month ago
#2
D is reversed (it is the opposite of B), so force exerted by the child on the ground.
A is the gravity, so the force exerted by the Earth on the COM of the chlid. Its the opposite of C.
Last edited by mqb2766; 1 month ago
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Thread starter 1 month ago
#3
(Original post by mqb2766)
D is reversed (it is the opposite of B), so force exerted by the child on the ground.
A is the gravity, so the force exerted by the Earth on the COM of the chlid. Its the opposite of C.
so you mean that, D and B are newton' third law pair forces and A and C are newton's third law pair forces.
but how?
lets say, if we keep a book on a table,
1) there's a force exerted by the book on the table
2) there's a force exerted by the table on the book
so 1 and 2 are newton's third law pair forces.
3) there's a force exerted by the earth on the book
4) there's a force exerted by the book on the earth
so 3 and 4 are newton's third law pair forces.

i don't still really do understand how A and C are newton's third law pair forces.
Last edited by Aleksander Krol; 1 month ago
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1 month ago
#4
(Original post by Aleksander Krol)
so you mean that, D and B are newton' third law pair forces and A and C are newton's third law pair forces.
but how?
lets say, if we keep a book on a table,
1) there's a force exerted by the book on the table
2) there's a force exerted by the table on the book
so 1 and 2 are newton's third law pair forces.
3) there's a force exerted by the earth on the book
4) there's a force exerted by the book on the earth
so 3 and 4 are newton's third law pair forces.

i don't still really do understand how A and C are newton's third law pair forces.
Gravity is non contact
Floor reaction is contact.

Just looking at the diagram, D&B and A&C must be pairs. A&C are graivty so act at the COM. D&B act at the point of contact.
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Thread starter 1 month ago
#5
(Original post by mqb2766)
Gravity is non contact
Floor reaction is contact.

Just looking at the diagram, D&B and A&C must be pairs. A&C are graivty so act at the COM. D&B act at the point of contact.
thanks!! it makes quite a lot sense now. <3<3
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1 month ago
#6
(Original post by Aleksander Krol)
thanks!! it makes quite a lot sense now. <3<3
Paired forces are usually of the same type so the gravitational attraction of the earth acting on the child is paired with the gravitational attraction of the child acting on the earth.
1
1 month ago
#7
(Original post by Joinedup)
Paired forces are usually of the same type so the gravitational attraction of the earth acting on the child is paired with the gravitational attraction of the child acting on the earth.
They're always of the same type - that's an important part of the third law that tends to get glossed over, but which makes questions like this much easier to deal with.
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